1. Field of the Invention
The present invention is directed to an antenna system for the reception of magnetic resonance signals with at least two resonant, individual antennas.
2. Description of the Prior Art
U.S. Pat. No. 5,471,142 discloses an antenna system of the above general type. The antenna system has a number of resonant individual antennas. Each individual antenna is connected to a pre-amplifier via a matching circuit. The signal path composed of the individual antenna, matching circuit and pre-amplifier is dimensioned such that the input impedance of the pre-amplifier attenuates the individual antenna. It is advantageous that the signal contributions of the individual antennas to the sum signal can be differently weighted. This, however, is at the expense of a high electronics outlay.
U.S. Pat. No. 4,721,913 discloses an antenna system that has two conductor loops arranged side-by-side in one plane, these being electrically connected in parallel and forming a single resonant circuit with a shared resonance capacitor. Magnetic transmission fields that are aligned perpendicular to the plane, however, induce currents in this arrangement that in turn distort the transmission field. Further, the signal contributions of the two conductor loops in the sum signal cannot be varied.
The antenna system disclosed in U.S. Pat. No. 4,816,765 has two series-connected conductor loops (butterfly antenna) and a single resonance capacitor, so that the two conductor loops form one resonant circuit. As in the antenna system disclosed in U.S. Pat. No. 4,721,913, the signal contributions of the two conductor loops in the sum signal cannot be varied.
The article by D. I. Hoult, xe2x80x9cFast Recovery with a Conventional Probexe2x80x9d, which appeared in Journal of Magnetic Resonance, Vol. 57, 1984, pp. 394-403, describes a circuit for noise-free antenna attenuation, as is desirable in the reception mode. An input impedance of a connected pre-amplifier is appropriately transformed at the signal terminal of the antenna so that it has an attenuating effect at this terminal.
An object of the present invention is to provide an antenna system with which signals of a number of individual antennas can be combined in order to form sensitivity distributions (magnetic field structures) that cannot be achieved with an individual antenna by itself. At the same time, an attenuation of the individual antennas should be possible on the basis of the input impedance of the pre-amplifier.
This object is achieved in an antenna system wherein the individual antennas are connected to a pre-amplifier via a combination circuit, with the combination circuit being fashioned such that signal contributions that can be emitted by the individual antennas exhibit a phase difference of 0xc2x0 or 180xc2x0 at the pre-amplifier. This passive interconnection of the individual antennas preceding the pre-amplifier allows the inherently known pre-amplifier decoupling of the individual antennas to be realized. The outlay for electronics remains low; nonetheless, the signal contributions of the individual antennas can be combined with differently weighted contributions.